1. Field of the Invention
The present invention relates generally to security systems, and more specifically to a multi channel radiometer that is expandable to increase the number of pixels and utilizes single pixel subassemblies to increase the functional test yield performance.
2. Description of the Prior Art
Security systems can be found at transportation centers such as airports, train stations or at other public facilities such as courthouses, government buildings, or public schools. One of the principal concerns of operators of security systems is the need to protect security personnel and other individuals (e.g. the general public) in the course of conducting a search of a person for concealed objects. The concealed objects that present a danger are weapons, explosives, contraband and other similar items.
Prior art security systems include metal or chemical residue detectors that require security personnel to be in proximity of the individual. One or more security personnel are required to conduct a hands-on or “wand-based” scan of an individual for whom the metal or chemical residue detector has generated an alarm. An inherent deficiency of this type of security system is the fact that it exposes not only the security personnel to danger, but also other individuals (e.g., travelers in an airport) in the vicinity of the security system to the dangers posed by such concealed objects. Accordingly, there is a need in the relevant art for a security system that has the ability to perform from a stand-off perspective so that security personnel and innocent by-standers are not exposed to any potential threat or danger.
The radiometer that is subject of this invention relates to millimeter wave engineering. This means that circuit elements must be scaled down such that passive elements and interconnects are fabricated on the same semiconductor substrate as the active devices to create a Monolithic Millimeter-wave Integrated Circuit (“MMIC”). The microscopic circuit elements are defined through photolithography from a scaled up mask to eliminate parasitics associated with component packages, leads and solder pads. The MMIC is smaller in mass and volume compared to conventional circuit assemblies. In addition, MMIC provides high performance.
Although there are advantages to using a MMIC, including low fabrication costs, there are also significant disadvantages. One significant disadvantage is that post fabricating tuning of circuit elements is not practical. More complex circuit designs require multiple iterations before acceptable performance is achieved and MMIC fabrication requires extremely long iteration times. Further, MMIC research and development requires specialized equipment such as wafer probes, probe stations, and wire bonders. Also, synthesized signal sources or spectrum analyzers makes circuit measurements difficult. Accordingly, there is a need in the art to improve the design and reliability of fabricated MMIC for radiometers to achieve acceptable performance of subassemblies.
Operating millimeter wave frequencies are between 30 and 300 GHz. The higher the frequency results in higher adsorption. The relatively high adsorption of the millimeter wave band makes long distance wireless communication not practical. However, the high adsorption of millimeter wave frequencies is attractive for other purposes. This includes the detection of concealed objects under an individual's clothing. Accordingly, radiometers have been developed using MMIC technology to detect millimeter wave frequencies for that purpose.
A millimeter wave imaging radiometer uses passive detection and measurement of electromagnetic radiation at millimeter wavelengths. The contrast in radiation between the surrounding background environment and individual undergoing a scan identifies concealed objects under clothing.
As part of the radiometer, a conical feedhorn is commonly used. Feedhorns are packed close together in the focal plane. The feedhorn defines the detector field of view and gives a tapered illumination of the scene. Maximum efficiency for the detection of a point source is achieved for a feedhorn diameter is close to 2Fλ where F is the focal ratio of the final optics and λ is the wavelength. To fully sample the image plane requires the feedhorn diameter and spacing to be 0.5Fλ. Feedhorns are readily understood in terms of their control of the beam coupling, are easy to fabricate, and offer good rejection of electromagnetic interference as the feedhorn and detector cavity act as a Faraday enclosure.
Typically, a large number of radiometer feedhorns are arranged into a focal plane array for contiguous imaging of an individual. A focal plane array is a two-dimensional array of detectors placed in the focal plane of a lens and used to collect information about an image positioned some distance from the lens. The choice of pixel architecture of a radiometer is critical to the design on a focal plane array where each pixel is coupled to the feedhorn.
Passive radiometers are sensitive to noise, therefore, MMIC low noise amplifiers (“LNA”) are critical components. Typically, a large number of LNAs are required for an imaging radiometer. Accordingly, a shortcoming of the prior art is that if an LNA fails as part of a subassembly of a series of LNAs, that results in the loss of the entire subassembly.
There have been attempts to improve radiometer imaging systems such that improved performance and reliability is achieved. By way of example, note U.S. Pat. No. 6,777,684 to Volkov et al., which discloses an apparatus that includes a source of radiation as part of the imaging system. A shortcoming of this prior art is that it uses an active radiation source thereby subjecting an individual to additional radiation exposure.
Another example is U.S. Pat. No. 7,132,648 to Vaidya, which is directed to improving the quality of a millimeter wave image using a set number of pixels. This is accomplished by compensating for the variation of the output signals from each channel receiving radiant energy emanating from a scene. However, the radiometer of Vaidya is not expandable to increase the number of pixels and thereby the resolution of an image.
U.S. Pat. No. 7,008,086 to Ammar discloses a radiometer that uses a combination of hybrid, low noise amplifiers (LNAs) in series. A shortcoming of this prior art is that it comprises a series of LNAs wherein one failed LNA of the series results in a completely failed subassembly and resultant increased fabrication costs.
It is desirable to provide a real time radiometer that provides the ability to increase or decrease the number of pixels and resolution of an image using the same housing and electrical hardware. There is a need for such a radiometer having a single MMIC LNA pixel design to increase the functional yield and thereby reduce fabrication costs.
There is also a need in the art for an improved security system that is non-invasive to the individual being searched for concealed objects.
Another need exists in the art to provide an improved security system that provides synchronized images from all angles of an individual's body to locate concealed objects.
Another need exists in the art to provide an improved security system that is easy for security personnel to operate and to synchronize.
Another need exists in the art to provide an improved security system to identify non-metallic concealed objects on an individual such as explosives.
It is, therefore, to the effective resolution of the aforementioned problems and shortcomings of the prior art that the present invention is directed.
However, in view of the prior art at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified needs could be fulfilled.